The main lens I have used is a zoom lens at 28 mm (for FF). It is NOT distortion free. Not at all. In my naivety I though - it does not matter, Autopano fixes it.

I have been doing lots of panos and it is very fun and it works good. But, with time comes higher demands and I see that the stitching is not perfect. Architecture images has not perfectly straight lines etc. And also some difficult panos almost was impossible to get accurate.

So - I started to do an investigation. Looking closer I see that the images are not really matching perfectly. Looking at individual images I see that the things in the pano moves around when switching image. Its the very good blending phase that saves it, that makes it look brilliant. But ... this is not enough for subjects that needs precision.

Next I found was that the focal length was strange and varied. Then I also saw that the lens parameters varied wildly. The lens has barrel distortion, and the focal length always were substantially longer. And ... this is the case if we do not use the center to determine focal length for a lens with barrel distortion. So - maybe the lens is not enough corrected and the outer part of the image is used for stitching. And, looking at the chosen stitching points, that was actually the case. The stitching points were not evenly distributed, they were, more or less, concentrated at a certain distance from the image centers. Which is necessary if you do not have the correct correction. Points that are not there will have too large distance and be removed.

Then I sat down and said - damned - I do not want to use this tool any more.

After recovering from that I made a test. I did straighten all my input pictures to some panos. So, now the images had no distortion. And then I did rerun my panos. And voila - all my problem were gone. The panos matched perfectly, and the stitching point were nicely spread out over the images.

So - therefor I say - you need to do something substantial about finding the correct lens parameters. If you have got the right parameters, then panoramas made with your tool will gain so much in quality.

I have found a solution that often works. You have to activate k3 in the distortion formula. Then it can handle my lens. Still, the result is slightly better when doing the correction before. But, it is good enough.